Which book by Nagarjuna provides an introduction to his teachings?

I actually think that the writings of Nagarjuna himself aren't necessarily the best place to start to study Madhyamaka. The Mulamadhyamakakarika is the sum total of his teachings on emptiness but it doesn't actually explicitly teach emptiness in the way you are looking for. For example, the opening line says:

Neither from itself nor from another, Nor from both, Nor without a cause, Does anything whatever, anywhere arise.

Which, taken at face value, is contrary to dependent origination. A lot of the Mulamadhyamakakarika is like that without explicitly teaching emptiness and the two truths. The real meaning of this first verse is that nothing arises from another, self, both, nor without a cause in terms of an intrinsic nature but the text itself doesn't explicitly clarify this. If you do want to study the Mulamadhyamakakarika, then I would recommend Jay Garfield's book The Fundamental Wisdom of the Middle Way because he also gives a commentary which is indispensable for a text like this.

Although it was not written by Nagarjuna, the Madhyamakavatara by Chandrakirti is a very good text that lays out the teachings of Madhyamaka, the philosophical tradition founded by Nagarjuna. I recommend the book The Emptiness of Emptiness that has a commentary and a translation of the Madhyamakavatara.

The guys over at Partially Examined Life have done a podcast on Nagarjuna. The format of the podcast is that they recommend a basic reading to accompany the show. The reading for this one was this by Dr Peter Della Santina so this might be a good way into Nagarjuna and his work on emptiness. Bear in mind though that the guys over at PEL read Kant for fun so basic for them might not be all that basic. I think though someone with a working knowledge of Buddhism should be OK.

As a side note - the podcast is very worthwhile if only to listen to their struggle as these clever guys twist around the mind bending stuff from Nagarjuna. Sadly the show has recently gone behind a paywall but if you read the PDF it might be worth while to spend a small amount of money getting the audio. You can preview for nothing to see if it suits.

Nagarjuna's Mulamadhyamakakarika is in response to the misguided interpretations of certain groups that attached teachings not originally expounded by the Gotama Buddha. Prof. Kalupahana's work in this area, in fact, his life's work was to bring to the fore that original teaching of the Buddha without the unnecessary metaphysics. So, it depends upon what the individual mind is disposed towards. If one's dispositions are inclined towards grasping metaphysics then Prof. Kalupahana's work and commentary on the Mulamadhyamakakarika would be most inappropriate. Garfield's and Inada's would suffice. On the other hand, if one leans toward an empirical approach to dhamma-vinaya,then, by all means, use this text.
This is not a text that can just be read. It must be studied and thought about, over and over, until the beautiful logic of it shines through. I think I have said enough.

Well, a great beginner level text attributed to Nagarjuna is Letter to a Friend. It's short and a really great read. But I wouldn't really say it offers insight to Nagarjuna's philosophy. It's more like a basic practical buddhism introduction.

However, like others have said, the MMK is his piece de resistance you could say. This is a rigorous and thorough investigation into the nature of reality by way of dialect with his opponents or naysayers you could call them. It's not easy by any means, so here's what I recommend you read before diving into the MMK.

There is a series of books called "The Foundation of Buddhist Thought". Volume 5 is dedicated to emptiness. This series is targeted towards a western audience and it's probably as simple as you're going to get for this topic:

http://www.amazon.com/Emptiness-Foundation-Buddhist-Thought-Volume/dp/086171511X

Bodhicitta Vivarana "A Commentary on the Awakening Mind" is both complete and accessible.

Sanskrit title: Bodhicittavivarana

Tibetan title: byang chub sems kyi 'grel pa

Homage to glorious Vajrasattva!
It has been stated:
Devoid of all real entities;
Utterly discarding all objects and subjects,
Such as aggregates, elements and sense-fields;
Due to sameness of selflessness of all phenomena,
One’s mind is primordially unborn;
It is in the nature of emptiness.

Just as the blessed Buddhas and the great bodhisattvas have generated the mind of
great awakening, I too shall, from now until I arrive at the heart of awakening,
generate the awakening mind in order that I may save those who are not saved, free
those who are not free, relieve those who are not relieved, and help thoroughly
transcend sorrow those who have not thoroughly transcended sorrow.

Those bodhisattvas who practice by means of the secret mantra, after having
generated awakening mind in terms of its conventional aspect in the form of an
aspiration, must [then] produce the ultimate awakening mind through the force of
meditative practice. I shall therefore explain its nature.

Bodhichitta Vivarana

It is a great text to study and reflect upon. If possible, try and spend time in nature when studying it because it really helps to get fresh air into your system and really get into a more "organic" setting

Here are some URLs for some good info on Nagarjuna and his teachings. Some great sites below, too. Dr. Berzin (Fulbright Scholar, etc.) was a translator for HH the Dalai Lama for a while and the late Ven. Tsenshab Serkong Rinpoche, a teacher of HH the Dalai Lama.

I. "Biography of Nagarjuna", [http://www.berzinarchives.com/web/en/archives/approaching_buddhism/teachers/lineage_masters/biography_nagarjuna.html]

II. "Course on Nagarjuna's Letter to a Friend", [http://www.berzinarchives.com/media/audio/en/podcast-hi/letter/] This was Nagarjuna's letter to a local King, which essentially lays out the graduated path to enlightenment. So, here is the foundation, the middle, and the end, so to speak, of Sutra-based teachings.

Hope that helps.

The word "introduction" may be out of place, but anyway his most important work would be the Mūlamadhyamakakārikā. A translation, along with an extensive commentary, is available in The Fundamental Wisdom of the Middle Way by Jay Garfield (here is a previously published commentary of chapters 1, 2 and 24). Bear in mind that this is a very, very difficult text to understand, even with the (indubitably necessary) help of commentators. Toward that end, let me point out Nāgārjuna's Madhyamaka: A Philosophical Introduction by Jan Westerhoff, which provides a synoptic overview of the arguments in Nāgārjuna's corpus concerning different philosophical problems with the aim of presenting an account of the whole of his philosophy (here is a helpful review of the book).